In recent years, relays which are used for an engine control unit for a car electric system are replaced with semiconductor devices such as power MOSFETs and so on to eliminate the contacts. As a recent device, IPD has been used in which protection functions such as functions of a current limitation circuit, an overheating detection circuit and an open load detection circuit are built in the power MOSFET. A self-diagnosis result can be transferred to a microcomputer on a control side. The IPD is requested to reduce the power consumption while suppressing the increase of a circuit area.
For example, Patent Literature 1 (JP H06-151740A) discloses a power semiconductor device. The power semiconductor device includes a semiconductor substrate, a plurality of power transistors, at least one diffusion layer and a trench separation oxide film. The semiconductor substrate has a first substrate of a high concentration and a substrate which is used for a common drain and a second substrate of a low concentration joined with the first substrate. The plurality of power transistors are formed on a main surface of the second substrate to divide the area. At least one diffusion layer is formed to be buried in the second substrate in correspondence to a boundary of the plurality of power transistors to divide the region of the plurality of power transistors. A trench separation oxide film is buried in the ditch which is formed in the second substrate in a depth to the first substrate in correspondence to the diffusion layer. The minimum voltage is set to the diffusion layer for the plurality of power transistors. Thus, the phenomenon can be prevented in which a parasitic transistor of the adjacent power transistors is turned on so that parasitic current flows from the source of one of the power transistors into the source of the other power transistor.